The present invention relates to a device for sharpening the teeth of motor chain saws. Users of motor saws are familiar with the necessity of sharpening the teeth on the chain at relatively frequent intervals to maintain satisfactory cutting, and are well aware that this procedure is difficult and time-consuming, or is a source of irritation. By its nature, the problem is particularly familiar to forestry workers.
According to normal standard, the chain saw teeth are filed purely manually by filing with a round file at a filing angle of approximately 30xc2x0.
Many devices have been developed to facilitate the sharpening operation, including control of the file alignment. In general, however, these known devices for controlling the forward and backward movement of the file, or even developing a rotary movement of the file, have not achieved widespread practical use.
Also, the use of motor grinding discs has been proposed, with an associtated fixture for holding the respective teeth of the chain. Apart from being troublesome, this method is even dangerous for the teeth themselves, because with a fast running grinding disc this easily results in the teeth being excessively heated, whereby their quality can be drastically reduced.
Chain teeth are in several respects very different from ordinary saw teeth, primarily in having not only an outwardly projecting flank portion, but also an outermost, bent-out top portion, which is to be sharpened from below such that the leading edges of both the flank and the top portion are kept sharp. Therefore, the tooth should be affected by a filing or grinding pressure in a direction away from the root of the tooth, in addition to the force against the flank itself. This influence is liable to cause the treated tooth carrying chain link to tilt outwardly from its position rested against the sword edge, and while this can be xe2x80x98feltxe2x80x99 and accepted by a manual filing it will nevertheless incur problems in connection with automatic sharpening, inasfar as some backing of the treated tooth is required.
Moreover, the tooth carrying chain links also have, at the end opposite to and operatively in front of the respective teeth, an outwardly protruding xe2x80x9criderxe2x80x9d, which is somewhat less protruding than the tooth and has the purpose of delimiting the intrusion of the tooth into the material being sawn and remove the resulting chips. These riders, arranged rather closely in front of the teeth, will to some degree prevent a grinding disc from entering the teeth in a fully ideal manner, which would be by way of a large disc rotating in a plane parallel with the chain carrying link in front of the tooth; in practice, however, it could be sufficient to operate with an inclined position of the plane of rotation, such that the disc edge will enter into the tooth with the disc just passing over the top of the rider.
Finally, the teeth of the saw chains appear with a much longer mutual distance than ordinary saw teeth. Normally there is a number of neutral chain links interposed between the consecutive tooth links, and since the consecutive teeth are layed out (or rather in) to opposite sides, the distance between two similarly oriented teeth will be correspondingly large. Although the chains are made with very high precision, unavoidable tolerances will cause small deviations of the distances referred to. The relevance of this observation will appear from the following.
Now, in connection with the present invention, it has been recognized that a more ideal shape of a rotary grinding or filing tool will be a file rod as first referred to, held against the tooth in exactly the same position as by conventional tooth filing, now only rounded up so as to form a relatively large circular element rising upwardly from the tooth and turned into an angular position relative to the saw sword corresponding to the desired filing angle, i.e. an angle of the magnitude 30xc2x0 from the direction perpendicularly to the sword. In this manner the filing ring element, in-the local area immediately in front of the tooth, will appear practically exactly as a conventional file rod, except for a slight curvature thereof, i.e. the ring element will in no way interfere with the said rider in front of the tooth.
According to an important aspect of the invention it will then also be possible to arrange for means to excert a holding pressure against the top of the rider belonging to the tooth being filed, whereby the tooth or the entire tooth link can be stabilized against the said tilting in response to the filing/grinding pressure being applied to the tooth, thus without any need of arranging for a support of the tooth at the opposite side thereof. According to the invention, such means for holding down the riders may be provided even on the ring element itself, viz. as a laterally projecting flange which, during the rotation of the ring element, will steadily and slidingly engage the top of the relevant rider and thus stabilize the tooth.
Already by these measures the invention, in practice, provides for a sharpening system that will be superior to previous proposals. However, the invention comprises a further and extremely important development, viz. based on the finding that the said circular filing/grinding element, without any significant change in the character of the local engagement with the single tooth, can be changed from a circular shape into three-dimensional screw shape, in which it is able to cooperate, simultaneously, with a number of consecutive teeth. As mentioned, the distance between the chain saw teeth is relatively long and the filing angle is relatively large, and it has been found that these two conditions enable the design of a xe2x80x9cscrew filexe2x80x9d having at each place a pitch angle corresponding to the desired filing angle and having a phase length equal to the distance between the teeth, without the diameter of this screw being extremely large or small compared with the chain or sword dimensions, this almost amounting to a xe2x80x9clucky coincidencexe2x80x9d. The screw file, in being rotated about an axis parallel with the moving direction of the teeth, will thus both sharpen the teeth and urge the teeth forwardly, that is rearwardly relative to their operational movement, in order to sharpen the teeth.
The required counter pressure on the teeth for the filing/grinding tool to-effect the desired result may be established already by the inherent resistance against the saw chain being moved along the sword.
Hereby an entirely novel perspective is encountered, viz. that the teeth of the saw chain can be sharpened by a continous movement, merely by a rotation of the filing/grinding screw element, when the latter is mounted in holding connection with the sword of the chain saw, with its axis of rotation held in parallel with the line of movement of the chain. The screw element may be driven at moderate speed, pressing itself against the front side of the teeth and thus forcing the chain rearwardly.
It is a further advantage that the direction of the filing/grinding movement is the preferred one, viz. with and not against the resulting sharp edges.
The sharpening screw element, just like the said circular filing or grinding element, may well be provided with the discussed flange member serving to hold down the riders, i.e. the tooth or teeth may be pushed rearwardly without any noticeable tilting, whereby a high quality sharpening is achievable by the fully automatic and continuous operation.
A novelty search based on the invention has revealed that it has earlier been proposed to sharpen ordinary saw teeth by means of rotating, screw shaped filing elements, see U.S. Pat. No. 570,732, published in 1896. However, the teaching of this document is impertinent to the sharpening of chain saw teeth, already because of the above-mentioned fundamental differences between ordinary saws and chain saws. By way of example, in connection with ordinary saw teeth it would not be possible to operate with a filing angle as large as 30xc2x0 with this technique, because between the teeth there would not be space enough for the required tool. Besides, the said document does not indicate the possibility of modifying the screw member into a pronounced high pitch member usable for chain saws.
Moreover, the said U.S. Pat. No. 570,732 does not take into account the discussed condition with respect to slight variations of the distance between the chain saw teeth. The disclosed apparatus is provided with two counter rotating screw files for engaging, at the same time, two saw teeth as laid out to opposite sides in the usual manner, whereby the two screw members will assist each other in pushing the saw blade (band-saw) in its reverse direction, filing or grinding the two teeth simultaneously. For achieving this result the distance between the two teeth or any pair of two such teeth has to be exactly the same as the operative distance between the two filing or grinding screws, as in case of a slightly larger distance one tooth will be heavily worked while the pressure on the other tooth is relieved and this tooth not worked effectively, while in case of a slightly shorter distance at least one of the teeth and/or one of the filing screws may be mechanically damaged.
On this background and in accordance with the present invention, the discussed system should be laid out so as to work only one tooth at a time and as to ensure that the screw element is ready behind the next (or rather second next) tooth when it gets out of engagement with the just worked tooth. Thereafter and preferably only after the working of all of the teeth laid out to the same side, the screw tool for working the remaining teeth is made operative and the first screw tool is made inoperative. In practice, the tools are simply interchanged.
Thus, in connection with the invention the problem is not the distance variations between the teeth of opposite layout, but those between the consecutive teeth of the same type.